An electronic music synthesizer is a keyboard instrument capable of producing sound whose characteristics, such as pitch, timbre, loudness, etc. can be controlled by the user. Such synthesizers have been commercially available for several years (e.g. Odyssey Model 2800 sold by Arp Instruments, Inc., Newton, Massachusetts) and are generally comprised of elements such as oscillators, amplifiers, filters, and envelope generators whose operating characteristics are controlled by DC voltage levels applied thereto by the user.
As was recognized in U.S. Pat. No. 3,715,444, prior art synthesizers have been limited in that they are single voiced or monophonic instruments; i.e. they are capable of playing only one note at a time. U.S. Pat. No. 3,715,444 is accordingly directed to a polyphonic musical instrument which includes multiple voice channels which can operate simultaneously to produce sound. However, the system disclosed in U.S. Pat. No. 3,715,444 requires complex multiple contact switches, employs a rather inflexible set of channel assignment rules, and is not easily expandable.
U.S. Pat. No. 3,733,955 is also directed to means for providing a synthesizer with more than one voice. However, the system disclosed in this patent is limited to two voices.